moa Question

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Guest, Dec 1, 2001.

  1. Guest

    Guest Guest

    new to the list
    ok i have a question here if a scope is calibrated in moa 1/4 moa = .261 @ 100 yds
    then at 1000 yds 1/4 moa would = 2.6179 in a perfect world acording to my calculations
    acording to my balistic program shooting 7 mm rem mag 160 gr sierra hpbt @ 2800 fps
    mv i should have approx 191 inches drop at 800 yds scope zeroed @ 200 yds so acording to my calculations @ 800 yds 1/4 moa = 2.094" i would need to adjust 91 clicks on the leupold vari x III 6-1/2-20
    to achieve a hit not taking into concideration wind also bullet rotational yaw
    so my question is am i on base here or am i in left field
    Thanks Steve
     
  2. Dave King

    Dave King Well-Known Member

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    Steve

    Welcome to Long Range Hunting.

    I believe you've got it!! 191 inches of drop at 800 yards = 91.2129894937 clicks on a Leupold scope calibrated at .25 MOA "Clicks". (I'd just go for the 91 "clicks" and skip the .2129894937 [​IMG] .)
     

  3. Guest

    Guest Guest

    Ok now that we are learning here how about bullet yaw due to rotational spin if the terminoligy is correct the curveture
    i guess it would be in y axis is there a formula velocity + barrel twist =
    or something like that
    thanks
    Steve
     
  4. Dave King

    Dave King Well-Known Member

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    Steve

    Sounds like you're heading for the 'fuzzy' areas.

    There's a bunch of stuff happening to the bullet as it flies. There are 'external' forces, wind, twigs, rain, etc.. there are also 'internal' forces variable on spin, bullet concentricity, jacket thickness variations, center of mass and center of rotation, there is also earth's rotation. It's fairly amazing that we can hit anything with a rifle at any distance greater than an inch or two beyond the muzzle [​IMG] .

    I believe the one you're heading for is commonly called 'spin drift' or I believe also known as Magnus effect/force. It's the same principle as a throwing a curve ball in baseball.

    Yaw as I understand it, is caused by forces acting on the bullets' gyroscopic stability, a low pressure area is trying to lift the front of the bullet as it flies forward while falling, this causes the bullet's nose to deviate 90 degrees from the lift force. I don't believe thhis would cause as much trajectory alteration as 'spin drift' as long as the bullet remains in a non-tumbling state.

    There is a lot of information available on the web about these effects and there are some knowledgable folks here too!! It's an area I don't know a lot about and will stay away from any significant technical aspects.

    At the distances I shoot and the degree of accuracy I accept and the nature of my targets it's not an immediate issue for me.

    There is a book that may cover a good deal of this info, it's authored by a fella that visits this board on occasion, I can only think of his posting name TriggerFifty. I'll look it up and be right back.. Dean Michaelis

    Here's a site that has some ingo on Dean, Warren and the long range rifles and shooting.
    http://www.cheyennetactical.com/rifles.htm
     
  5. gkgeske

    gkgeske Member

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    The problem with theory is that between us and the long range target there is a great deal of 'real world': conditions that usually cannot be seen, and theory cannot explain, that will move the bullet one way or another. Take a sighter shot, and adjust the equiptment and theories accordingly! [​IMG]
     
  6. Dave King

    Dave King Well-Known Member

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    Gwahir

    Welcome to Long Range Hunting

    Steve

    As you can see by the the responses from Gwahir and Darryl the general method of correcting is to fire a spotter or two and correct. This is the method used by most hunters and is a tried and true method.

    But, if you're interested in the theory side of the issue(s) there are folks that have a very good understanding of one shot hits at tremendous ranges. These folks don't have the option of firing a spotter round. There are methods to calculate coriolis, angle of slope (even small angles are significant at extreme ranges), spin drift, multivector winds, etc. This is not a better method for the hunter, it's just an alternate method and probably beyond the time and training most folks would would care to invest.
     
  7. Guest

    Guest Guest

    ok i was wrong about yaw
    what you mentioned about spin drift is what i ment
    that is what i am trying to figure out is a drop chart
    well i havent even started on the real world stuff the space between here & there
    wind rain vectering wind up hill down hill

    the rotation of the earth ?
    whats that got to do with a 800 to 1000 yrd shot
    oh boy looks like im going to get to burn up a few barrels

    the sighter shot would probably be the best thing to do
    but i doubt if wily will hang around even at a 800 yrd away and 100 yds off of him
    maybe im wrong i havent tried it yet

    about a half mile is about all i can shoot here i okla. besides the 7rem mag is running out of gas out past that
     
  8. Darryl Cassel

    Darryl Cassel Well-Known Member

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    Hello Gwahir

    I agree with your statement.

    Even target shooters take sighter shoots to adjust accordingly.Why would a LR hunter not take one or two also and let his spotter with bigeyes make the bullet impact call?

    At the ranges we shoot, you don't have time to try figure Yaw, earth rotation and such.
    A good drop chart with the elevation, temperature, velocity and other pertinent information along with a good hunting partner with excellent optics is what is needed in longrange hunting.

    You will NEVER do it alone if you can't see the hits in excess of 1000 yards. Just too many variables out there to contend with.

    Have the best Rangefinder you can afford, apply that to your pre run ballistics program that you have checked out before the season began, and count on your spotter to make the call. Just like a sniper "team" does it.

    Darryl Cassel
     
  9. Dave King

    Dave King Well-Known Member

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    Steve

    For 99% of folks, myself included, all the things OTHER than verification of a good drop chart is not essential or warranted. (That statement will get the Purist's excited!)

    For a target the size of a coyote, I'd have a good (verified) drop chart that included some temperature variations.

    Use a chonograph and get an accurate velocity for your load. Get/use a good Ballistics program and get a ballistic printout based on your velocity, bullet BC, terrain altitude and normal temperature and humidity (these are listed in rough order of importance, most to least). Get the printout in 50 yard increments at a minimum, you can extrapolate to a smaller amount from that. Get to an area where you can set up some targets to your desired maximum range, do this on a calm day with the temperature range you've selected. ZERO THE RIFLE AT 100 YARDS..... NOW, estimate the wind speed and apply windage correction (with your rifle I'd start at .5 MOA for every 100 yards for a 10mph full value wind) use the drop chart and put the 200 yard data on the scope and fire on the 200 yard target, record the horizontal and vertical distance from your point of aim and the center of the shot group for that target. (Fire your shots when the wind conditions match the conditions of the previous shot.)
    Repeat the above procedure until you're completely frustrated [​IMG] or you've completed the drop chart verification to your maximum range.
    The MORE important data here is the vertical displacement information of your groups. Adjust your drop chart by using the theoretical chart and the actual data from your verification, you should now have a good drop chart for that rifle and load.
    The wind data is good in that you'll have an idea of future wind calls and adjusting for the wind. If it get's too confusing forget about adjusting for the wind and concentrate only on the drop data. In a perfect world, AND FOR THE PURPOSES OF THIS EXERCISE, all horizontal displacement is wind if you have a good 100 yard zero.

    Don't worry about the Earth's rotation, spin drift, or rain. Concentrate on marksmanship fundamentals, drop charts, wind and shooting on angles.

    MOST animals don't know what a gun is, they don't recognize the report of a rifle as danger, especially if it's a good distance away. Animals that are routinely shot AT learn some danger signs, the sight of a person is about the #1 danger sign, stay out of sight. Some learn that ANYTHING out of the ordinary is a signal to vacate the area, this includes loud noises. But they sometimes also learn what is considered a safe distance, shooting noises at 800 yards is probably not something they have learned to associate with danger.
    I'd try the sighter shot as many folks here use, the coyotes may not feel threatened and you may even be able to entice them into coming closer. After a while I'd say that for ranges of 600 to 800 yards you'll be able to skip the sighter and get first round hits but you'll need to have good charts and be observant.
     
  10. gkgeske

    gkgeske Member

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    I heard a wonderful story from a friend about a sighter shot. He had done this a lot with elk, and they stood well at 1k or so. Anyway, in this case he saw four black bear in the same tree at about 800 yds and laid in a sighter 'far enough' off to the side. All four bear turned loose of the tree at exactly the split second the bullet arrived, and they were doing 40 mph before they hit the ground!! No, I don't suppose old Yote would stand well for such activities. [​IMG]
     
  11. Darryl Cassel

    Darryl Cassel Well-Known Member

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    Hello

    We killed bear with the sighter shot method and the bear did not run at all. This one was at 750 yards. There was others but, the yardage on this one was close to the one in the story you mentioned.

    The point is you must be far enough off with your sighter round for animals like the bear and the Coyote.
    What looks like 100 yards off from a 1000 yards away may not be.

    The sighter round is mainly for the windage adjustment if you have shot your rifle enough and know the clicks at the various ranges. Again an accurate drop chart is a must.

    As mentioned before, the sound is usually not a factor at longrange especially with deer and elk. Obviously if your first shot goes in close to or below the animal (I'm still talking about deer and elk), he will normally run about 25 to 50 yards, stop and look back at what that impact noise was. Sometimes they will walk up and stick their nose in the impact hole. They are not alarmed at all (In most cases).

    The bear and cotote are a bit more alarmed at CLOSE noise. Make your sighter round at least 100 yards away (or further) and you should have no problems.

    Hope that helped a bit.
    DC

    [ 12-03-2001: Message edited by: Darryl Cassel ]
     
  12. Guest

    Guest Guest

    thanks dave gwahir & darrel
    you guys are top notch
    well what kind of wind meters are you all
    using

    as far as yotes i can call then almost into my lap some days
    some days they dont respond
    but i think it would be exciting if i could
    have the tools & capability to do the job out that far

    the 408 cheyenne tac seems to be quite the shooter are there any info on chamber dims.
    load development or any thing to the general public

    thanks steve
     
  13. 308

    308 Well-Known Member

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    Steve Web, I don't think that the Leupolds are calibrated in moa per say. From the info I've found and limited testing with mine and from reading Dan Lilja's articles about longrange hunting I'm pretty sure all of the vari x III's are .250 per click.
    I think the only Leupolds that are actually pure moa or .262 per click are the 30mm MK 4 scopes. I'm sure some of these folks here can say which is correct. 308